1. Field of the Invention
The present invention relates to a control apparatus of a hydraulically operated vehicular transmission having at least two speed gears including a low speed gear and a high speed gear.
2. Description of the Related Art
This type control apparatus is provided with a shift valve to be freely changed over between a low speed position for establishing a low speed stage and a high speed position for establishing a high speed stage. The shift valve is changed over according to a predetermined speed change characteristic in which an engine load and a vehicle speed are set as parameters, so that automatic gear change can be conducted.
Control apparatuses can be divided into two main classes. One is an electronic control type control apparatus in which a shift valve is changed over by using an electromagnetic valve controlled by a controller into which signals are inputted from an engine load sensor and a vehicle speed sensor. The other is a purely hydraulic type control apparatus which uses a first hydraulic signal output means for outputting a first hydraulic signal according to an engine load and also uses a second hydraulic signal output means for outputting a second hydraulic signal according to a vehicle speed, and a shift valve is changed over onto a low speed position side when the shift valve is pushed according to the first hydraulic signal and an energizing means and also the shift valve is changed over onto a high speed position side when the shift valve is pushed according to the second hydraulic signal. Recently, the electronic control type control apparatus is generally used, however, the purely hydraulic type control apparatus is also used for the reason of reducing the cost.
In this connection, when an operator conducts a down-shift operation, for example, when an operator changes over from an automatic gear changing range to a low speed holding range, it is necessary for him to change over a shift valve to a low speed position so as to conduct a down-shift operation from a high speed gear to a low speed gear. For example, in the purely hydraulic type control apparatus shown in Japanese Examined Patent Publication No. 5-27785, a valve section arranged in a hydraulic oil path for inputting the second hydraulic signal into the shift valve is formed into a manually operated valve. Here, the above hydraulic oil path is opened in the automatic gear changing range, however, the hydraulic oil path is shut off by the valve section in the low speed holding range, so that the input of the second hydraulic signal into the shift valve can be stopped.
In the above purely hydraulic type control apparatus of the related art, when a driver conducts down-shift operation and a vehicle speed at that time is higher than that corresponding to the down-shifted gear, an engine overruns. In order to prevent the engine from overrunning, it is necessary to provide another hydraulic circuit or an electronic control type prevention device.